10.17862/cranfield.rd.7223942.v1James BremnerDistributed gas sensing using microstructured optical fibres2018Cranfield Online Research Data (CORD)gasspectroscopymultiplexedCranfieldDN20182018-10-22 09:51:01articlehttps://cord.cranfield.ac.uk/articles/Distributed_gas_sensing_using_microstructured_optical_fibres/7223942<p>3 Minute Thesis presented at the Cranfield Doctoral Network Annual Event 2018.</p><p><br></p><p>With a
global warming potential 34 times that of CO2, as well as an explosion hazard,
methane monitoring is of interest. Current
technology requires the placement of a large number of sensors to cover an
area. We propose a distributed technique to allow the use of a single measurement
device to cover multiple sampling sites. Tunable Diode Laser Spectroscopy has
been combined with optical fibre coupling of gas cells to permit a large number of cells to be
interrogated simultaneously. Techniques to multiplex a number of TDLS cells
together have been developed including time-division-multiplexing, Optical
Time Domain Reflectometry and use of multiple fibres, each with its own
detector.Range Resolved Interferometry (RRI) is an interferometric signal
processing technique that has been used for position sensing and vibrometry.
The principle of operation is the sinusoidal modulation of the emission
wavelength of a diode laser which is input into an interferometer. The signal
is demodulated to give amplitude information within each interferometer.
Signals from different interferometers can be distinguished based on the
optical path length difference, enabling interrogation of all measurement
channels simultaneously.Here, a combination of RRI and absorption spectroscopy
is performed to multiplex two gas cells arranged to form two interferometers
and to recover gas concentrations from the two cells independently. The
configuration is two 1m path length single-pass gas cells connected via
different lengths of optical fibre in a nested Mach-Zehnder interferometer
along with a common reference arm. The two gas cells were filled with
different concentrations of methane. A
distributed feedback diode laser was modulated with a ramp and sinusoidal wave
form. The ramp sweeps the laser emission frequency across a methane absorption
line at 1651nm, while the sinusoidal modulation generates interference
patterns.RRI was used to recover the amplitude from the two gas cells and
thereby the concentration of methane.</p>